This invention relates to layered materials incorporating expanded PTFE membrane which are useful as filter media for filtering solids from fluid streams, such as, for example, industrial gas streams, commercial and residential vacuum cleaner dust streams, and the like.
The removal of particulates from a gas stream has long been a practice in a variety of industrial and consumer fields. Conventional means for filtering particulates and the like from gas streams include, but are not limited to, filter bags, filter tubes and filter cartridges. For convenience herein, the term xe2x80x9cfilter elementxe2x80x9d will be used to refer collectively to these types of filtration means.
Conventional filtration techniques utilize the filter media to stop particles through the depth of the element, and as the particles build up in and/or on the element, the filtration efficiency of the element is increased. After an amount of dust has caked on the surface of the filter element, the flow rate of gas through the element is reduced to a level where the bulk dust cake must be removed from the element, typically by some form of agitation, such as vibration or the like.
Conventional filter elements are typically constructed from felt and/or fabric media made from a variety of materials, including polyesters, polypropylenes, aramids, glasses and fluoropolymers. Selection of the type of media used is typically based on the gas stream with which the filter element comes in contact, the operating conditions of the system and the type of particulates being filtered.
A significant development in the area of particle filtration was achieved when expanded PTFE membrane was incorporated as a surface laminate on conventional filter elements. One example is taught in U.S. Pat. No. 4,878,930, directed to a filter cartridge for removing particles of dust from a stream of moving gas or air. Preferred filter media for the cartridge are felt or fabric composites containing a layer of porous expanded polytetrafluoroethylene membrane.
Use of the expanded PTFE membrane greatly enhanced the performance of filter elements because the particles collected on the surface of the expanded PTFE, rather than in the depth of the elements as was occurring in the absence of the expanded PTFE layer. Several significant advantages were observed with these filter elements. First, the filtration efficiency of the elements was high immediately from the outset of the filtration process, and it was not necessary to xe2x80x9cbuild upxe2x80x9d a cake of particles to achieve high efficiency. Second, the elements lasted longer because particles were not getting into the backing fabric of the element and rubbing on the fibers to wear them out. Third, for cleanable systems, the cleaning energy needed to clean the particle cakes off of the elements was lower because the surface of the membrane was smooth and had a lower surface energy.
A variety of vacuum cleaners are presently available that can effectively pick up dry material alone, or both dry and wet materials. These devices are found in a variety of forms, but all generally comprise a large holding tank with a suction unit mounted on the tank. Dry and/or wet materials are drawn through a hose into the holding tank during suction.
In the case of dry material pick-up, it is important that dust drawn into the tank of a vacuum cleaner is not blown into the air outside the tank through the vacuum exhaust. In order to avoid this situation, all of these vacuums are sold with some form of filter mounted between the tank and the exhaust to contain dry dust contamination within the vacuum""s tank. In the case of wet/dry vacuum cleaners (referred to for convenience as xe2x80x9cwet/dry vacsxe2x80x9d), one common complaint of users is that the filters are not suitable for use with wet materials. When the typical wet/dry vac filters are exposed to water or even wet materials (e.g., wet leaves), the water tends to wet-out the paper filters and quickly destroys them under the harsh conditions of vacuuming. Many of these problems have been overcome by the use of wet/dry vac filters employing a hydrophobic and air permeable filter material, such as an expanded polytetrafluoroethylene (PTFE). These improved wet/dry vac filter cartridges are available from W. L. Gore and Associates, Inc., sold under the trademark CLEANSTREAM(copyright). These filters upon installation have been demonstrated to provide a filtration efficiency of 60 to 99%, or more, for 0.3 micron particles, and preferably an efficiency of 99.7 to 99.97 or more.
Over time, particle impingement, whether on membrane or non-membrane filter media can severely affect filter performance. Direct particle impingement and dust impaction has common effects on most porous air filter media regardless of material or construction. For example, whether the filter media comprises spunbond or meltblown polyester, polypropylene, polyethylene, fiberglass, microfiberglass, or ePTFE membrane, particle impingement can result in reduced permeability due to dust impregnation (both short term and permanent), accelerated degradation of the filter media characterized by fraying, efficiency loss or puncture and reduced cleanability as a result of permanent dust impregnation. Moreover, particularly for such high efficiency filter media as ePTFE membrane and microfiberglass, these media are subject to further special problems in that they are particularly susceptible to damage as a result of particle impingement. These ePTFE membranes are designed to provide exceptionally high air filtration efficiencies, but are relatively less abrasion resistant than other lower efficiency media. Therefore the intended function can be seriously compromised by efficiency degradation caused by particle impact abrasion. Even a small amount of media damage can cause the filter to operate below required performance levels, necessitating filter replacement, which is both expensive and time consuming.
The layered materials of this invention are designed to solve these problems.
The present invention is an improved layered structure suitable as a filter media for gas filtration systems, such as for industrial gas streams and commercial or residential vacuum cleaners. The improved media comprises a support layer such as a non-woven polyester, polypropylene, polyethylene, fiberglass or microfiberglass, bonded to one side of a porous expanded PTFE membrane and a protective surface pattern bonded to the other side of the expanded PTFE membrane. The protective surface pattern protects the expanded PTFE against damage due to impingement of collected particles during operation of the gas filtration system. Thus, the novel filter media is capable of withstanding abrasion induced degradation, while maintaining good filtration efficiency during use. Accordingly, enhanced durability of the filter media is achieved without compromising cleanability.
In a preferred embodiment, the filter media comprises a support layer bonded to a porous expanded PTFE membrane, and further comprises a protective surface pattern comprising a screen material, as described in more detail herein, bonded by lamination to the porous expanded PTFE.